SMT chip refers to a series of technological processes processed on the basis of PCB. PCB (Printed Circuit Board) is a printed circuit board.SMT YesSurface Mount Technology(Surface Mounted Technology) is the most popular technology and process in the electronic assembly industry.
The surface mount technology (SMT) of electronic circuits is called surface mount technology or surface mount technology.It is a kind of surface assembled components (SMC/SMD for short) without pins or short leads installed on thePrinted circuit boardPrinted Circuit Board (PCB) or the surface of other substrates, which are welded and assembled by reflow soldering or dip soldering.
In general, the electronic products we use are designed by PCB plus various electronic components such as capacitors and resistors according to the designed circuit diagram, so all kinds of electrical appliances need different SMT chip processing processes to process[1]。
Electronic products pursue miniaturization, but previously used perforated plug-in components can no longer be reduced.At present, the integrated circuits (ICs) used have no perforated components, especially large-scale and highly integrated ICs, so surface mount components have to be used.With mass production and production automation, the manufacturer should produce high-quality products with low cost and high output to meet customer needs and strengthen market competitiveness. The development of integrated circuit (IC), the diversified application of semiconductor materials and the development of electronic components are imperative.It can be imagined that when the production process of international CPU and image processing device manufacturers such as Intel and amd is refined to 20 nm, SMTSurface Mount TechnologyAnd the development of technology.
The advantages of SMT chip processing: high assembly density, small size and light weight of electronic products, the size and weight of SMT components are only about 1/10 of that of traditional plug-in components, high reliability, strong anti vibration ability, and low solder joint defect rate.Generally, after using SMT, the volume of electronic products can be reduced by 40%~60%, and the weight can be reduced by 60%~80%.SMT chip has good high-frequency characteristics, reduces electromagnetic and radio frequency interference, is easy to realize automation, improves production efficiency, and can reduce costs by 30%~50%.Save materials, energy, equipment, manpower, time, etc.
It is precisely because of the complexity of SMT chip processing process flow that many SMT chip processing factories have emerged, specializing in SMT chip processing. In Shenzhen, thanks to the booming development of the electronic industry, SMT chip processing has achieved the prosperity of an industry.
technological process
Announce
edit
The basic process elements of SMT include: screen printing (or dispensing), mounting (curing), reflow welding, cleaning, testing, and repair.
1. Silk screen printing: It is used to leak solder paste or paster onto the bonding pad of PCB to prepare for the welding of components.The equipment used is silk screen printing machine（Screen printing machine）, onSMT production lineThe front end of.
2. Glue dispensing: it is to drop glue on the fixed position of the PCB board, and its main function is to fix components on the PCB board.The equipment used isDispenser , located at the front end of the SMT production line or behind the detection equipment.
3. Mounting: it is used to accurately install surface mounted components on the fixed position of PCB.The equipment used is a mounting machine, which is located behind the screen printing machine in the SMT production line.
4. Curing: Its function is to melt the patch adhesive, so that the surface mounted components and PCB board are firmly bonded together.The equipment used is a curing furnace, which is located behind the SMT mounter in the SMT production line.
5. Reflow soldering: its function is to melt the solder paste, so that the surface assembled components andPCB boardThey are firmly bonded together.The equipment used is a reflow furnace, which is located behind the SMT mounter in the SMT production line.
6. Cleaning: It is used to remove the harmful welding residues such as flux on the assembled PCB board.The equipment used is a cleaning machine, which can be located at any position, online or offline.
7. Inspection: it is used to inspect the welding quality and assembly quality of the assembled PCB.The equipment used includes magnifier, microscope, on-line tester (ICT)Flying needle tester, Automatic Optical Inspection (AOI), X-RAY detection system, function tester, etc.The position can be configured in the appropriate place of the production line according to the needs of detection.
8. Rework: it is used to detect faultsPCB boardRework.The tools used are soldering iron, repair workstation, etc., which can be configured at any position in the production line.
A: Incoming material detection=>A-side screen solder paste of PCB (spot paste adhesive)=>B-side screen solder paste of patch PCB (spot paste adhesive)=>patch=>drying=>reflow soldering (it is better to only use B-side=>cleaning=>detection=>repair).
This process is applicable to reflow soldering on the A side of PCB and wave soldering on the B side.This process should be used when there are only SOTs or SOIC (28) pins below in the SMDs assembled on the B side of PCB.
A: Incoming material inspection=>PCB B-side spot paster=>paster=>curing=>flip=>PCB A-side plug-in=>wave soldering=>cleaning=>inspection=>repair
Paste first and insert later, applicable to the case where SMD components are more than separation components.
B: Incoming material inspection=>A side plug-in of PCB (pin bending)=>Flipping=>B side spot patch adhesive of PCB=>Patch=>Curing=>Flipping=>Wave soldering=>Cleaning=>Inspection=>Repair
Plug first and paste later, which is applicable to the case where there are more separation elements than SMD elements.
D: Incoming material inspection=>PCB B-side spot paster=>paster=>curing=>flip=>PCB A-side screen solder paste=>paster=>A-side reflow soldering=>plug-in=>B-side wave soldering=>cleaning=>inspection=>repair A-side mixed packaging, B-side paste packaging.Paste SMD on both sides first, reflow soldering, then insert, wave soldering E: incoming material detection=>PCB B-side silk screen solder paste (spot patch adhesive)=>patch=>drying (curing)=>reflow soldering=>flip=>PCB A-side silk screen solder paste=>patch=>drying=reflow soldering 1 (local welding can be used)=>plug-in=>wave soldering 2 (if there are few plug-in components, manual welding can be used)=>Cleaning=>testing=>repairing A-side mounting and B-side mixing.
Double side assembly process
A: Incoming material detection, A-side screen printing solder paste (spot paste) of PCB, mounting, drying (curing), A-side reflow soldering, cleaning, and flip;Screen printing solder paste (spot paste) on the B-side of PCB, mounting, drying, reflow soldering (it is better to clean, detect and repair the B-side only)
This process is applicable when large SMDs such as PLCC are mounted on both sides of the PCB.
B: Incoming material detection, A-side screen printing solder paste (spot paste) of PCB, mounting, drying (curing), A-side reflow soldering, cleaning, and flip;This process is applicable to reflow on the A side of PCB.
Such film lines are generally printed on PET with silver paste.Paste on such film linesElectronic componentsThere are two process methods, one is called the traditional process method, namely the three glue method (red glue, silver glue, enveloping glue) or the two glue method (silver glue, enveloping glue), and the other is called the new process, namely the one glue method - just as the name implies, it is possible to paste electronic components with one glue instead of three or two glues.The key of this new process is to use a new type of conductive adhesive, which completely has the conductive and technological properties of solder paste;Fully compatible with currentSMTThe solder paste brushing method does not need to add any equipment.
selection
Announce
edit
The selection and design of surface mounted components is a key part of the overall product design. The designer determines the electrical performance and function of components in the system structure and detailed circuit design phase, and determines the packaging form and structure of surface mounted components in the SMT design phase according to the specific conditions of equipment and process and the overall design requirements.The surface mounted solder joint is both a mechanical connection point and an electrical connection point. Reasonable selection will improve thePCB designDensity, producibility, testability and reliability all have a decisive impact.
Surface mounted components have no difference in function from plug-in components, but the difference lies in the packaging of components.Surface mounted packages must withstand high temperature during welding. Components and substrates must have matchingCoefficient of thermal expansion。These factors must be considered comprehensively in product design.
The advantages of choosing a suitable package are:
1) Effectively save PCB area;
2) Provide better electrical performance;
3) It can protect the interior of components from moisture and other environmental influences;
4) Provide good communication;
5) Helps dissipate heat and facilitates delivery and testing[2]。
Selection of surface mounted components
Surface mounted components are classified into active and passive components.According to the pin shape, it can be divided into "gull wing" type and "J" type.The selection of components is described in this classification below.
Passive component
Passive componentMainly including single pieceCeramic capacitor、Tantalum capacitorAnd thick film resistors, which are rectangular or cylindrical in shape.cylindricalPassive componentIt is called "MELF". When reflow welding is adopted, rolling is easy to occur. Special pad design is required, which should be avoided in general.The rectangular passive components, called CHIP chip components, are widely used in various electronic products due to their small size, light weight, good impact and shock resistance of antibiotics, and low parasitic loss.In order to obtain good solderability, the electroplating of nickel base barrier layer must be selected.
Active device
There are two major types of surface mount chip carriers: ceramics and plastics.
The advantages of ceramic chip packaging are:
1) Good air tightness, good protection for internal structure;
2) The signal path is short, and the parasitic parameters, noise, and delay characteristics are significantly improved;
3) Reduce power consumption.
The disadvantage is that there is no pin to absorb the stress generated when the solder paste melts, and the mismatch of CTE between the package and the substrate can lead to solder joint cracking during welding.The most commonly used ceramic wafer carrier is lead-free ceramic wafer carrier LCCC.
Plastic packaging is widely used in the production of military and civilian products, with good cost performance.Its packaging forms are divided into: small shape transistor SOT;Small form factor integrated circuitSOIC；Plastic leaded chip carrier PLCC；Small form factor J packaging;Plastic flatpack PQFP.
In order to effectively reduce the PCB area, SOICs with less than 20 pins, PLCCs with 20-84 pins, and PQFPs with more than 84 pins are preferred under the same device function and performance.
Reduce faults
Announce
edit
The manufacturing process, handling and printed circuit assembly (PCA) testing will make the package bear a lotMechanical stress, causing a fault.As the grid array package becomes larger and larger, it becomes more difficult to set the security level for these steps.
For many years, the use of monotonic bending point test method is a typical feature of packaging. This test is described in IPC/JEDEC-9702 "Monotonic Bending Characteristics of Horizontal Interconnection of Boards".This test method describespcbThe breaking strength of the horizontal interconnection under bending load.However, this test method cannot determine the maximum allowable tension.
For the manufacturing process and assembly process, especially for lead-free PCA, one of the challenges it faces is the inability to directly measure the stress on the solder joint.The most widely used measure to describe the risk of interconnected components is the tension of the printed circuit board adjacent to the component, which is described in IPC/JEDEC-9704 Guide for Strain Testing of Printed Circuit Boards.
Years agoIntelThe company realized this problem and started to develop a different test strategy to reproduce the worst bending situation in practice.Other companies such as Hewlett Packard also realized the benefits of other testing methods and began to consider similar ideas with Intel.As more and more chip manufacturers and customers realize that the determination of tension limits used to minimize mechanical failures during manufacturing, handling and testing is of great value, this method has aroused more and more interest.
With the expansion of the use of lead-free equipment, the interest of users is growing;Because many users face quality problems.
With the increasing interest of all parties, IPC feels it is necessary to help other companies develop various kinds of products that can ensureBGAA test method that is undamaged during manufacturing and testing.This work was conducted by the IPC 6-10d SMT Attachment Reliability Test Method Working Group andJEDECJC-14.1 Packaging Equipment Reliability Test Methods Subcommittee has jointly carried out the work, which has been completed.
This test method specifies eight contact points arranged in a circular array.The PCA with a BGA installed in the center of the printed circuit board is placed as follows: the component is installed on the support pin face down, and the load is applied on the back of the BGA.The strain gauge shall be placed adjacent to the component according to the recommended gauge layout of IPC/JEDEC-9704.
The PCA will be bent to the relevant tension levels, and the failure analysis can determine the extent of damage caused by bending to these tension levels.The tension level without damage can be determined by iterative method, which is the tension limit.
Packaging materials
Announce
edit
Generally, the packaging materials are plastic and ceramic. The heat dissipation part of the component may be composed of metal. The pins of the component are divided into lead and lead-free[3]。
